Differential amplifiers amplify (e.g., augment, increase, etc.) differential signals (e.g., signals measured as equal and opposite between two nodes). The factor by which the signals are amplified, called the amplifier's gain, is a measure of the ability of the amplifier to increase the power or amplitude of the signal from the input to the output. The gain is finite, and depends on the frequency of the input signal. At low frequency, the gain is maximum, usually decreases exponentially with increasing frequency, and has a value of one at the frequency commonly referred to as the unity-gain frequency. Generally, for an amplifier implemented in a given technology, the product of the amplifier's gain and maximum bandwidth is constant. Consequently, the design of the amplifier typically involves compromising between high gain and high bandwidth.
When the input to the amplifier changes too fast, the amplifier's slew rate, which is the maximum rate of change of the output voltage per unit of time (e.g., expressed as volt per second) slows the output. Changing of the output voltage over time for large input steps is generally referred to as “stewing.” The slew rate of the amplifier, in general, decreases as gain increases, leading to a trade-off between gain and slew rate (e.g., higher the gain, lower the slew rate and vice versa). A high slew rate is generally a desirable characteristic of the amplifier; likewise, a high gain is also a desirable characteristics of the amplifier; however, amplifiers typically cannot be designed to provide both high slew rate and high gain.
One mechanism to achieve a high gain, high slew rate amplifier involves adding a positive current feedback circuit to a standard differential amplifier with resistive loads. The loop gain of the feedback circuit is configured to be smaller than unity and is controlled so as to ensure the stability of the entire unit. The load resistors are connected between a supply pole and the outputs of the differential amplifier and mounted in parallel with two current sources which apply their current feedback to the outputs. Another implementation of a high gain, high slew rate amplifier includes a primary amplifier combined with a secondary high power amplifier, which is activated only during brief periods, during which a very high slew rate is required.
In yet another implementation, a buffer circuit having both high gain and high slew rate is implemented using a high gain, low slew rate amplifier and a switch network with three separate phases of operation. During the first phase, the output of the amplifier is isolated from the load to allow the amplifier's output voltage to more quickly reach its final voltage level (e.g., achieving high slew rate). During the second phase, the switch network couples the amplifier output lead to the load input lead where the amplifier drives the voltage at the load's input lead to a voltage substantially equal to the voltage of the input signal (e.g., achieving high gain). During the third phase, the switch network isolates the amplifier from the load and couples the load's input lead to a source of ground potential to quickly slew the load's input lead to ground potential.